Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 605 insertions, 0 deletions

diff --git a/Documentation/bpf/standardization/instruction-set.rst b/Documentation/bpf/standardization/instruction-set.rst
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+.. contents::
+.. sectnum::
+
+=======================================
+BPF Instruction Set Specification, v1.0
+=======================================
+
+This document specifies version 1.0 of the BPF instruction set.
+
+Documentation conventions
+=========================
+
+For brevity and consistency, this document refers to families
+of types using a shorthand syntax and refers to several expository,
+mnemonic functions when describing the semantics of instructions.
+The range of valid values for those types and the semantics of those
+functions are defined in the following subsections.
+
+Types
+-----
+This document refers to integer types with the notation `SN` to specify
+a type's signedness (`S`) and bit width (`N`), respectively.
+
+.. table:: Meaning of signedness notation.
+
+  ==== =========
+  `S`  Meaning
+  ==== =========
+  `u`  unsigned
+  `s`  signed
+  ==== =========
+
+.. table:: Meaning of bit-width notation.
+
+  ===== =========
+  `N`   Bit width
+  ===== =========
+  `8`   8 bits
+  `16`  16 bits
+  `32`  32 bits
+  `64`  64 bits
+  `128` 128 bits
+  ===== =========
+
+For example, `u32` is a type whose valid values are all the 32-bit unsigned
+numbers and `s16` is a types whose valid values are all the 16-bit signed
+numbers.
+
+Functions
+---------
+* `htobe16`: Takes an unsigned 16-bit number in host-endian format and
+  returns the equivalent number as an unsigned 16-bit number in big-endian
+  format.
+* `htobe32`: Takes an unsigned 32-bit number in host-endian format and
+  returns the equivalent number as an unsigned 32-bit number in big-endian
+  format.
+* `htobe64`: Takes an unsigned 64-bit number in host-endian format and
+  returns the equivalent number as an unsigned 64-bit number in big-endian
+  format.
+* `htole16`: Takes an unsigned 16-bit number in host-endian format and
+  returns the equivalent number as an unsigned 16-bit number in little-endian
+  format.
+* `htole32`: Takes an unsigned 32-bit number in host-endian format and
+  returns the equivalent number as an unsigned 32-bit number in little-endian
+  format.
+* `htole64`: Takes an unsigned 64-bit number in host-endian format and
+  returns the equivalent number as an unsigned 64-bit number in little-endian
+  format.
+* `bswap16`: Takes an unsigned 16-bit number in either big- or little-endian
+  format and returns the equivalent number with the same bit width but
+  opposite endianness.
+* `bswap32`: Takes an unsigned 32-bit number in either big- or little-endian
+  format and returns the equivalent number with the same bit width but
+  opposite endianness.
+* `bswap64`: Takes an unsigned 64-bit number in either big- or little-endian
+  format and returns the equivalent number with the same bit width but
+  opposite endianness.
+
+
+Definitions
+-----------
+
+.. glossary::
+
+  Sign Extend
+    To `sign extend an` ``X`` `-bit number, A, to a` ``Y`` `-bit number, B  ,` means to
+
+    #. Copy all ``X`` bits from `A` to the lower ``X`` bits of `B`.
+    #. Set the value of the remaining ``Y`` - ``X`` bits of `B` to the value of
+       the  most-significant bit of `A`.
+
+.. admonition:: Example
+
+  Sign extend an 8-bit number ``A`` to a 16-bit number ``B`` on a big-endian platform:
+  ::
+
+    A:          10000110
+    B: 11111111 10000110
+
+Instruction encoding
+====================
+
+BPF has two instruction encodings:
+
+* the basic instruction encoding, which uses 64 bits to encode an instruction
+* the wide instruction encoding, which appends a second 64-bit immediate (i.e.,
+  constant) value after the basic instruction for a total of 128 bits.
+
+The fields conforming an encoded basic instruction are stored in the
+following order::
+
+  opcode:8 src_reg:4 dst_reg:4 offset:16 imm:32 // In little-endian BPF.
+  opcode:8 dst_reg:4 src_reg:4 offset:16 imm:32 // In big-endian BPF.
+
+**imm**
+  signed integer immediate value
+
+**offset**
+  signed integer offset used with pointer arithmetic
+
+**src_reg**
+  the source register number (0-10), except where otherwise specified
+  (`64-bit immediate instructions`_ reuse this field for other purposes)
+
+**dst_reg**
+  destination register number (0-10)
+
+**opcode**
+  operation to perform
+
+Note that the contents of multi-byte fields ('imm' and 'offset') are
+stored using big-endian byte ordering in big-endian BPF and
+little-endian byte ordering in little-endian BPF.
+
+For example::
+
+  opcode                  offset imm          assembly
+         src_reg dst_reg
+  07     0       1        00 00  44 33 22 11  r1 += 0x11223344 // little
+         dst_reg src_reg
+  07     1       0        00 00  11 22 33 44  r1 += 0x11223344 // big
+
+Note that most instructions do not use all of the fields.
+Unused fields shall be cleared to zero.
+
+As discussed below in `64-bit immediate instructions`_, a 64-bit immediate
+instruction uses a 64-bit immediate value that is constructed as follows.
+The 64 bits following the basic instruction contain a pseudo instruction
+using the same format but with opcode, dst_reg, src_reg, and offset all set to zero,
+and imm containing the high 32 bits of the immediate value.
+
+This is depicted in the following figure::
+
+        basic_instruction
+  .-----------------------------.
+  |                             |
+  code:8 regs:8 offset:16 imm:32 unused:32 imm:32
+                                 |              |
+                                 '--------------'
+                                pseudo instruction
+
+Thus the 64-bit immediate value is constructed as follows:
+
+  imm64 = (next_imm << 32) | imm
+
+where 'next_imm' refers to the imm value of the pseudo instruction
+following the basic instruction.  The unused bytes in the pseudo
+instruction are reserved and shall be cleared to zero.
+
+Instruction classes
+-------------------
+
+The three LSB bits of the 'opcode' field store the instruction class:
+
+=========  =====  ===============================  ===================================
+class      value  description                      reference
+=========  =====  ===============================  ===================================
+BPF_LD     0x00   non-standard load operations     `Load and store instructions`_
+BPF_LDX    0x01   load into register operations    `Load and store instructions`_
+BPF_ST     0x02   store from immediate operations  `Load and store instructions`_
+BPF_STX    0x03   store from register operations   `Load and store instructions`_
+BPF_ALU    0x04   32-bit arithmetic operations     `Arithmetic and jump instructions`_
+BPF_JMP    0x05   64-bit jump operations           `Arithmetic and jump instructions`_
+BPF_JMP32  0x06   32-bit jump operations           `Arithmetic and jump instructions`_
+BPF_ALU64  0x07   64-bit arithmetic operations     `Arithmetic and jump instructions`_
+=========  =====  ===============================  ===================================
+
+Arithmetic and jump instructions
+================================
+
+For arithmetic and jump instructions (``BPF_ALU``, ``BPF_ALU64``, ``BPF_JMP`` and
+``BPF_JMP32``), the 8-bit 'opcode' field is divided into three parts:
+
+==============  ======  =================
+4 bits (MSB)    1 bit   3 bits (LSB)
+==============  ======  =================
+code            source  instruction class
+==============  ======  =================
+
+**code**
+  the operation code, whose meaning varies by instruction class
+
+**source**
+  the source operand location, which unless otherwise specified is one of:
+
+  ======  =====  ==============================================
+  source  value  description
+  ======  =====  ==============================================
+  BPF_K   0x00   use 32-bit 'imm' value as source operand
+  BPF_X   0x08   use 'src_reg' register value as source operand
+  ======  =====  ==============================================
+
+**instruction class**
+  the instruction class (see `Instruction classes`_)
+
+Arithmetic instructions
+-----------------------
+
+``BPF_ALU`` uses 32-bit wide operands while ``BPF_ALU64`` uses 64-bit wide operands for
+otherwise identical operations.
+The 'code' field encodes the operation as below, where 'src' and 'dst' refer
+to the values of the source and destination registers, respectively.
+
+=========  =====  =======  ==========================================================
+code       value  offset   description
+=========  =====  =======  ==========================================================
+BPF_ADD    0x00   0        dst += src
+BPF_SUB    0x10   0        dst -= src
+BPF_MUL    0x20   0        dst \*= src
+BPF_DIV    0x30   0        dst = (src != 0) ? (dst / src) : 0
+BPF_SDIV   0x30   1        dst = (src != 0) ? (dst s/ src) : 0
+BPF_OR     0x40   0        dst \|= src
+BPF_AND    0x50   0        dst &= src
+BPF_LSH    0x60   0        dst <<= (src & mask)
+BPF_RSH    0x70   0        dst >>= (src & mask)
+BPF_NEG    0x80   0        dst = -dst
+BPF_MOD    0x90   0        dst = (src != 0) ? (dst % src) : dst
+BPF_SMOD   0x90   1        dst = (src != 0) ? (dst s% src) : dst
+BPF_XOR    0xa0   0        dst ^= src
+BPF_MOV    0xb0   0        dst = src
+BPF_MOVSX  0xb0   8/16/32  dst = (s8,s16,s32)src
+BPF_ARSH   0xc0   0        :term:`sign extending<Sign Extend>` dst >>= (src & mask)
+BPF_END    0xd0   0        byte swap operations (see `Byte swap instructions`_ below)
+=========  =====  =======  ==========================================================
+
+Underflow and overflow are allowed during arithmetic operations, meaning
+the 64-bit or 32-bit value will wrap. If BPF program execution would
+result in division by zero, the destination register is instead set to zero.
+If execution would result in modulo by zero, for ``BPF_ALU64`` the value of
+the destination register is unchanged whereas for ``BPF_ALU`` the upper
+32 bits of the destination register are zeroed.
+
+``BPF_ADD | BPF_X | BPF_ALU`` means::
+
+  dst = (u32) ((u32) dst + (u32) src)
+
+where '(u32)' indicates that the upper 32 bits are zeroed.
+
+``BPF_ADD | BPF_X | BPF_ALU64`` means::
+
+  dst = dst + src
+
+``BPF_XOR | BPF_K | BPF_ALU`` means::
+
+  dst = (u32) dst ^ (u32) imm32
+
+``BPF_XOR | BPF_K | BPF_ALU64`` means::
+
+  dst = dst ^ imm32
+
+Note that most instructions have instruction offset of 0. Only three instructions
+(``BPF_SDIV``, ``BPF_SMOD``, ``BPF_MOVSX``) have a non-zero offset.
+
+The division and modulo operations support both unsigned and signed flavors.
+
+For unsigned operations (``BPF_DIV`` and ``BPF_MOD``), for ``BPF_ALU``,
+'imm' is interpreted as a 32-bit unsigned value. For ``BPF_ALU64``,
+'imm' is first :term:`sign extended<Sign Extend>` from 32 to 64 bits, and then
+interpreted as a 64-bit unsigned value.
+
+For signed operations (``BPF_SDIV`` and ``BPF_SMOD``), for ``BPF_ALU``,
+'imm' is interpreted as a 32-bit signed value. For ``BPF_ALU64``, 'imm'
+is first :term:`sign extended<Sign Extend>` from 32 to 64 bits, and then
+interpreted as a 64-bit signed value.
+
+The ``BPF_MOVSX`` instruction does a move operation with sign extension.
+``BPF_ALU | BPF_MOVSX`` :term:`sign extends<Sign Extend>` 8-bit and 16-bit operands into 32
+bit operands, and zeroes the remaining upper 32 bits.
+``BPF_ALU64 | BPF_MOVSX`` :term:`sign extends<Sign Extend>` 8-bit, 16-bit, and 32-bit
+operands into 64 bit operands.
+
+Shift operations use a mask of 0x3F (63) for 64-bit operations and 0x1F (31)
+for 32-bit operations.
+
+Byte swap instructions
+----------------------
+
+The byte swap instructions use instruction classes of ``BPF_ALU`` and ``BPF_ALU64``
+and a 4-bit 'code' field of ``BPF_END``.
+
+The byte swap instructions operate on the destination register
+only and do not use a separate source register or immediate value.
+
+For ``BPF_ALU``, the 1-bit source operand field in the opcode is used to
+select what byte order the operation converts from or to. For
+``BPF_ALU64``, the 1-bit source operand field in the opcode is reserved
+and must be set to 0.
+
+=========  =========  =====  =================================================
+class      source     value  description
+=========  =========  =====  =================================================
+BPF_ALU    BPF_TO_LE  0x00   convert between host byte order and little endian
+BPF_ALU    BPF_TO_BE  0x08   convert between host byte order and big endian
+BPF_ALU64  Reserved   0x00   do byte swap unconditionally
+=========  =========  =====  =================================================
+
+The 'imm' field encodes the width of the swap operations.  The following widths
+are supported: 16, 32 and 64.
+
+Examples:
+
+``BPF_ALU | BPF_TO_LE | BPF_END`` with imm = 16/32/64 means::
+
+  dst = htole16(dst)
+  dst = htole32(dst)
+  dst = htole64(dst)
+
+``BPF_ALU | BPF_TO_BE | BPF_END`` with imm = 16/32/64 means::
+
+  dst = htobe16(dst)
+  dst = htobe32(dst)
+  dst = htobe64(dst)
+
+``BPF_ALU64 | BPF_TO_LE | BPF_END`` with imm = 16/32/64 means::
+
+  dst = bswap16(dst)
+  dst = bswap32(dst)
+  dst = bswap64(dst)
+
+Jump instructions
+-----------------
+
+``BPF_JMP32`` uses 32-bit wide operands while ``BPF_JMP`` uses 64-bit wide operands for
+otherwise identical operations.
+The 'code' field encodes the operation as below:
+
+========  =====  ===  ===========================================  =========================================
+code      value  src  description                                  notes
+========  =====  ===  ===========================================  =========================================
+BPF_JA    0x0    0x0  PC += offset                                 BPF_JMP class
+BPF_JA    0x0    0x0  PC += imm                                    BPF_JMP32 class
+BPF_JEQ   0x1    any  PC += offset if dst == src
+BPF_JGT   0x2    any  PC += offset if dst > src                    unsigned
+BPF_JGE   0x3    any  PC += offset if dst >= src                   unsigned
+BPF_JSET  0x4    any  PC += offset if dst & src
+BPF_JNE   0x5    any  PC += offset if dst != src
+BPF_JSGT  0x6    any  PC += offset if dst > src                    signed
+BPF_JSGE  0x7    any  PC += offset if dst >= src                   signed
+BPF_CALL  0x8    0x0  call helper function by address              see `Helper functions`_
+BPF_CALL  0x8    0x1  call PC += imm                               see `Program-local functions`_
+BPF_CALL  0x8    0x2  call helper function by BTF ID               see `Helper functions`_
+BPF_EXIT  0x9    0x0  return                                       BPF_JMP only
+BPF_JLT   0xa    any  PC += offset if dst < src                    unsigned
+BPF_JLE   0xb    any  PC += offset if dst <= src                   unsigned
+BPF_JSLT  0xc    any  PC += offset if dst < src                    signed
+BPF_JSLE  0xd    any  PC += offset if dst <= src                   signed
+========  =====  ===  ===========================================  =========================================
+
+The BPF program needs to store the return value into register R0 before doing a
+``BPF_EXIT``.
+
+Example:
+
+``BPF_JSGE | BPF_X | BPF_JMP32`` (0x7e) means::
+
+  if (s32)dst s>= (s32)src goto +offset
+
+where 's>=' indicates a signed '>=' comparison.
+
+``BPF_JA | BPF_K | BPF_JMP32`` (0x06) means::
+
+  gotol +imm
+
+where 'imm' means the branch offset comes from insn 'imm' field.
+
+Note that there are two flavors of ``BPF_JA`` instructions. The
+``BPF_JMP`` class permits a 16-bit jump offset specified by the 'offset'
+field, whereas the ``BPF_JMP32`` class permits a 32-bit jump offset
+specified by the 'imm' field. A > 16-bit conditional jump may be
+converted to a < 16-bit conditional jump plus a 32-bit unconditional
+jump.
+
+Helper functions
+~~~~~~~~~~~~~~~~
+
+Helper functions are a concept whereby BPF programs can call into a
+set of function calls exposed by the underlying platform.
+
+Historically, each helper function was identified by an address
+encoded in the imm field.  The available helper functions may differ
+for each program type, but address values are unique across all program types.
+
+Platforms that support the BPF Type Format (BTF) support identifying
+a helper function by a BTF ID encoded in the imm field, where the BTF ID
+identifies the helper name and type.
+
+Program-local functions
+~~~~~~~~~~~~~~~~~~~~~~~
+Program-local functions are functions exposed by the same BPF program as the
+caller, and are referenced by offset from the call instruction, similar to
+``BPF_JA``.  The offset is encoded in the imm field of the call instruction.
+A ``BPF_EXIT`` within the program-local function will return to the caller.
+
+Load and store instructions
+===========================
+
+For load and store instructions (``BPF_LD``, ``BPF_LDX``, ``BPF_ST``, and ``BPF_STX``), the
+8-bit 'opcode' field is divided as:
+
+============  ======  =================
+3 bits (MSB)  2 bits  3 bits (LSB)
+============  ======  =================
+mode          size    instruction class
+============  ======  =================
+
+The mode modifier is one of:
+
+  =============  =====  ====================================  =============
+  mode modifier  value  description                           reference
+  =============  =====  ====================================  =============
+  BPF_IMM        0x00   64-bit immediate instructions         `64-bit immediate instructions`_
+  BPF_ABS        0x20   legacy BPF packet access (absolute)   `Legacy BPF Packet access instructions`_
+  BPF_IND        0x40   legacy BPF packet access (indirect)   `Legacy BPF Packet access instructions`_
+  BPF_MEM        0x60   regular load and store operations     `Regular load and store operations`_
+  BPF_MEMSX      0x80   sign-extension load operations        `Sign-extension load operations`_
+  BPF_ATOMIC     0xc0   atomic operations                     `Atomic operations`_
+  =============  =====  ====================================  =============
+
+The size modifier is one of:
+
+  =============  =====  =====================
+  size modifier  value  description
+  =============  =====  =====================
+  BPF_W          0x00   word        (4 bytes)
+  BPF_H          0x08   half word   (2 bytes)
+  BPF_B          0x10   byte
+  BPF_DW         0x18   double word (8 bytes)
+  =============  =====  =====================
+
+Regular load and store operations
+---------------------------------
+
+The ``BPF_MEM`` mode modifier is used to encode regular load and store
+instructions that transfer data between a register and memory.
+
+``BPF_MEM | <size> | BPF_STX`` means::
+
+  *(size *) (dst + offset) = src
+
+``BPF_MEM | <size> | BPF_ST`` means::
+
+  *(size *) (dst + offset) = imm32
+
+``BPF_MEM | <size> | BPF_LDX`` means::
+
+  dst = *(unsigned size *) (src + offset)
+
+Where size is one of: ``BPF_B``, ``BPF_H``, ``BPF_W``, or ``BPF_DW`` and
+'unsigned size' is one of u8, u16, u32 or u64.
+
+Sign-extension load operations
+------------------------------
+
+The ``BPF_MEMSX`` mode modifier is used to encode :term:`sign-extension<Sign Extend>` load
+instructions that transfer data between a register and memory.
+
+``BPF_MEMSX | <size> | BPF_LDX`` means::
+
+  dst = *(signed size *) (src + offset)
+
+Where size is one of: ``BPF_B``, ``BPF_H`` or ``BPF_W``, and
+'signed size' is one of s8, s16 or s32.
+
+Atomic operations
+-----------------
+
+Atomic operations are operations that operate on memory and can not be
+interrupted or corrupted by other access to the same memory region
+by other BPF programs or means outside of this specification.
+
+All atomic operations supported by BPF are encoded as store operations
+that use the ``BPF_ATOMIC`` mode modifier as follows:
+
+* ``BPF_ATOMIC | BPF_W | BPF_STX`` for 32-bit operations
+* ``BPF_ATOMIC | BPF_DW | BPF_STX`` for 64-bit operations
+* 8-bit and 16-bit wide atomic operations are not supported.
+
+The 'imm' field is used to encode the actual atomic operation.
+Simple atomic operation use a subset of the values defined to encode
+arithmetic operations in the 'imm' field to encode the atomic operation:
+
+========  =====  ===========
+imm       value  description
+========  =====  ===========
+BPF_ADD   0x00   atomic add
+BPF_OR    0x40   atomic or
+BPF_AND   0x50   atomic and
+BPF_XOR   0xa0   atomic xor
+========  =====  ===========
+
+
+``BPF_ATOMIC | BPF_W  | BPF_STX`` with 'imm' = BPF_ADD means::
+
+  *(u32 *)(dst + offset) += src
+
+``BPF_ATOMIC | BPF_DW | BPF_STX`` with 'imm' = BPF ADD means::
+
+  *(u64 *)(dst + offset) += src
+
+In addition to the simple atomic operations, there also is a modifier and
+two complex atomic operations:
+
+===========  ================  ===========================
+imm          value             description
+===========  ================  ===========================
+BPF_FETCH    0x01              modifier: return old value
+BPF_XCHG     0xe0 | BPF_FETCH  atomic exchange
+BPF_CMPXCHG  0xf0 | BPF_FETCH  atomic compare and exchange
+===========  ================  ===========================
+
+The ``BPF_FETCH`` modifier is optional for simple atomic operations, and
+always set for the complex atomic operations.  If the ``BPF_FETCH`` flag
+is set, then the operation also overwrites ``src`` with the value that
+was in memory before it was modified.
+
+The ``BPF_XCHG`` operation atomically exchanges ``src`` with the value
+addressed by ``dst + offset``.
+
+The ``BPF_CMPXCHG`` operation atomically compares the value addressed by
+``dst + offset`` with ``R0``. If they match, the value addressed by
+``dst + offset`` is replaced with ``src``. In either case, the
+value that was at ``dst + offset`` before the operation is zero-extended
+and loaded back to ``R0``.
+
+64-bit immediate instructions
+-----------------------------
+
+Instructions with the ``BPF_IMM`` 'mode' modifier use the wide instruction
+encoding defined in `Instruction encoding`_, and use the 'src' field of the
+basic instruction to hold an opcode subtype.
+
+The following table defines a set of ``BPF_IMM | BPF_DW | BPF_LD`` instructions
+with opcode subtypes in the 'src' field, using new terms such as "map"
+defined further below:
+
+=========================  ======  ===  =========================================  ===========  ==============
+opcode construction        opcode  src  pseudocode                                 imm type     dst type
+=========================  ======  ===  =========================================  ===========  ==============
+BPF_IMM | BPF_DW | BPF_LD  0x18    0x0  dst = imm64                                integer      integer
+BPF_IMM | BPF_DW | BPF_LD  0x18    0x1  dst = map_by_fd(imm)                       map fd       map
+BPF_IMM | BPF_DW | BPF_LD  0x18    0x2  dst = map_val(map_by_fd(imm)) + next_imm   map fd       data pointer
+BPF_IMM | BPF_DW | BPF_LD  0x18    0x3  dst = var_addr(imm)                        variable id  data pointer
+BPF_IMM | BPF_DW | BPF_LD  0x18    0x4  dst = code_addr(imm)                       integer      code pointer
+BPF_IMM | BPF_DW | BPF_LD  0x18    0x5  dst = map_by_idx(imm)                      map index    map
+BPF_IMM | BPF_DW | BPF_LD  0x18    0x6  dst = map_val(map_by_idx(imm)) + next_imm  map index    data pointer
+=========================  ======  ===  =========================================  ===========  ==============
+
+where
+
+* map_by_fd(imm) means to convert a 32-bit file descriptor into an address of a map (see `Maps`_)
+* map_by_idx(imm) means to convert a 32-bit index into an address of a map
+* map_val(map) gets the address of the first value in a given map
+* var_addr(imm) gets the address of a platform variable (see `Platform Variables`_) with a given id
+* code_addr(imm) gets the address of the instruction at a specified relative offset in number of (64-bit) instructions
+* the 'imm type' can be used by disassemblers for display
+* the 'dst type' can be used for verification and JIT compilation purposes
+
+Maps
+~~~~
+
+Maps are shared memory regions accessible by BPF programs on some platforms.
+A map can have various semantics as defined in a separate document, and may or
+may not have a single contiguous memory region, but the 'map_val(map)' is
+currently only defined for maps that do have a single contiguous memory region.
+
+Each map can have a file descriptor (fd) if supported by the platform, where
+'map_by_fd(imm)' means to get the map with the specified file descriptor. Each
+BPF program can also be defined to use a set of maps associated with the
+program at load time, and 'map_by_idx(imm)' means to get the map with the given
+index in the set associated with the BPF program containing the instruction.
+
+Platform Variables
+~~~~~~~~~~~~~~~~~~
+
+Platform variables are memory regions, identified by integer ids, exposed by
+the runtime and accessible by BPF programs on some platforms.  The
+'var_addr(imm)' operation means to get the address of the memory region
+identified by the given id.
+
+Legacy BPF Packet access instructions
+-------------------------------------
+
+BPF previously introduced special instructions for access to packet data that were
+carried over from classic BPF. However, these instructions are
+deprecated and should no longer be used.